1. Technical Field
The present invention relates to optical modules and optical communications devices that are used for optical communications.
2. Related Art
A variety of modes of optical modules are known. One of them is an optical module having a structure in which a part of light emitted from a light emitting element is received by a monitoring light receiving element, thereby monitoring the amount of the light. This type of optical module performs feedback control of a driving current for the optical element based on a current outputted from the monitoring light receiving element according to the amount of received light, which makes it possible to maintain the amount of light emitted from the light emitting element constant without regard to changes in the surrounding environment or deterioration of the optical element with the laps of time. In particular, when a vertical cavity surface emitting laser (VSCEL) whose light emission amount greatly changes according to the ambient temperature is used as a light emitting element, a feedback control (APC: automatic power control) to increase the driving current with an increase in the ambient temperature becomes more important in order to maintain the amount of light emission constant. In this instance, it is desirous as an ideal condition that the amount of light emitted from a light emitting element and coupled to an optical fiber through, for example, a lens or the like (the amount of light coupled with the fiber) is proportional to the amount of light received by a monitoring light receiving element (the amount of light monitored). For this reason, the temperature characteristics of the respective members such as the monitoring light receiving element, the lens and the like are reduced as much as possible such that the proportional relation between the amount of light monitored and the amount of light coupled to the fiber can be maintained. In this connection, examples of related art are described in Japanese Laid-open Patent Applications JP-A-2004-72072 and JP-A-10-65189.
However, even when the temperature characteristics of the respective member are suppressed, the emission angle of emission light of the VCSEL changes according to the ambient temperature, which causes a phenomenon in which the amount of light incident upon the monitoring light receiving element reduces with an elevation of the ambient temperature. As a solution to this phenomenon, for example, the area of the light receiving section of the monitoring light receiving element may be made sufficiently large. However, such a measure has a limitation due to restrictions on mounting, and is contradictory to the demand for miniaturization of optical modules.
Also, when the feedback control (APC) described above is performed, the operational temperature range of the optical module depends on characteristics of the light emitting element. Concretely, the light output of the light emitting element such as a VCSEL lowers as the ambient temperature becomes higher, such that, if its light output is set too high at room temperature (for example, about 25° C.), its light output cannot be maintained, when the ambient temperature rises, at the same level as the level at room temperature. In contrast, if the light output is set to a relatively low level at room temperature, the desired light output can be secured even when the ambient temperature rises, whereby the operational temperature range of the optical module can be widened. However, when the light output is set to a relatively low level, the waveform quality of optical signal at the time of high-speed modulation driving of the light emitting element may be deteriorated. This problem becomes more conspicuous as the ambient temperature lowers. Accordingly, in reality, the light output cannot be set to a level that is too low. In other words, the operational temperature range of the optical module cannot be widened.
In this respect, in order to widen the operational temperature range of an optical module, a variety of methods, such as, a method to improve the temperature characteristic of a light emitting element itself a method to provide a system to maintain constant the temperature of a light emitting element itself and the like, are considered. However, the element structure may require a complex design in order to improve the temperature characteristic of a light emitting element and to realize high-speed modulation. For example, related art is described in Japanese Laid-open Patent Applications JP-A-6-5916 and JP-A-2004-214311. Furthermore, in order to maintain the temperature of a light emitting element itself constant, a mechanism, such as, for example, a heat sink or the like may be needed (for example, see Japanese Laid-open Patent Application JP-A-2003-318488). However, such a method causes problems, such as, an increased number of components of the optical module, violation of the demand for miniaturization of the optical module, and the like.